Radio broadcast .. (1922-30)

260 Radio Broadcast Chemical polarity indicators are made in the form of armored glass tubes with terminals at each end. They contain a solution of starch and iodide of potassium which, decomposed by the passage of current, turns blue at the plus terminal. Direct current voltmeters, with a magnetic, motor movement, have the terminals marked -f (positive) and — (negative). If they are improperly connected (the positive lead to the minus post) the needle will swing to the left of zero; and a reading will only be obtained when connected positive to plus and negative to minus. Wires carrying direct current, carefully separated in slightly salty water, will decompose the solution into its composite gases, Hydrogen and Oxygen. As water, H^, contains twice the amount of hydrogen (H2) as of oxygen (O), the greater number of the hydrogen bubbles, which rise about the negative wire, indicate its polarity. A battery should be placed on charge when its voltage, under a normal load, drops to 1.7, and it may be considered fully charged when the current has been kept on for an hour and a half after it commences gassing freely, if, during that period, there is no further rise in either the voltage or the specific gravity. The voltage during the last hours of charge will vary, depending on the age of the battery from 2.3 to 2.5 (on open circuit 2.2 volts) and the specific gravity from 1.280 to 1.300. The specific gravity is the density or weight of one cubic centimeter of the electrolyte as compared with a similar quantity of chemically pure water. Sulphuric acid, of which the electrolyte is a twenty to twenty-five per cent, solution, is heavier than water, and during discharge some of it is absorbed into the plates, leaving the solution so much lighter. Charging is fundamentally a process of T* n *ii rf 'fi wf HI ~" FIG. I forcing the acid out of the plates back into the solution, boosting up the specific gravity until the electrolyte regains every bit of its original strength. Hence, the only loss in the electrolyte is occasioned by the evaporation ot water, which should be replenished (preferably distilled water) from time to time, maintaining the level of the solution one half to three quarters of an inch above the top of the plates. All changes in the electrolyte should be made in accordance with hydrometer readings, and only when the battery is fully charged. Directions for the maintenance and care of different batteries are furnished by their respective manufacturers, and the experimenter should be thoroughly familiar with those covering his own battery. VACUUM TUBES What are the structural differences that make some vacuum tubes more suitable for radio frequency amplification, and others suitable for audio-frequency amplification and for detection? — G. K., NEW LONDON, CONN. THE only structural differences between types of modern vacuum tubes are those differentiating power or transmitting tubes from low power receiving bulbs. The transmitting bulbs have larger elements in proportion to the amount of energy they control, with different spacing suited to the requirements of insulation, and a higher vacuum. In low capacity receiving tubes (capacity in the condenser sense), which include practically all the present day bulbs used for detection and amplification, the structural details may be identical, whether used for radio or audio-frequency amplification. However, until a few years ago, when tube construction became a more exact science, all tubes possessed a comparatively high capacity, due principally to the design and placing of the elements and leads. As the tubes were imperfectly evacuated, it is also possible that the presence of air or gas, with the corresponding dielectric constant, may have increased this capacity. Due to this condenser effect, pre-war tubes could not be used for radio frequency transformer amplification on short waves. The capacity of a tube is virtually shunted across the primary of the amplifying transformer, which, if the transformer is of the radio frequency type, will boost the wave, as will any condenser across an inductance. Thus, on short waves, where the addition of even small capacities has a comparatively large effect on wavelength, radio-frequency amplification was very inefficient, for little transference of energy could be effected on the few turns of wire to which the resonant transformer was limited. As mentioned above, there is to-day no structural difference between radio-frequency and audio-frequency amplifying bulbs. However, detector tubes have a lower vacuum than those designed for amplification. Several effects combine to make the lowvacuum audion more sensitive to weak grid impulses, and they are therefore more